The present invention pertains to X-ray fluorography apparatus, and in particular, to automatic brightness control systems for such apparatus.
During a fluoroscopic examination of a patient, an X-ray image is produced on the screen of a video monitor. To produce this image, the X-rays passing through the patient are picked up by an image intensifier tube, which produces a visible light image corresponding to the X-ray image. A video camera receives the visible light image from the intensifier tube and produces a video signal for the monitor, which displays the X-ray image.
As the X-ray beam scans different portions of the patient, the brightness of the video image will change due to variations in the attenuation of the X-ray beam as it passes through different thicknesses and densities of body tissue. In order to compensate for these variations in video brightness, different automatic compensation systems have been devised. One such system is described in U.S. Pat. No. 4,703,496 entitled "Automatic X-Ray Imager Brightness Control" and issued to the same assignee as the present invention. When this X-ray apparatus is operated in the fluorography mode, the luminance of the picture elements in each video image field is averaged to produce a signal representing the average brightness of the X-ray image. This average brightness signal then is used to control the excitation of the X-ray tube, thereby varying the X-ray dose to maintain the video image brightness substantially constant. Heretofore, such automatic brightness control systems were analog in nature in that the video signal section and the image brightness averager consisted of analog signal processing circuits.
With the advent of digital image storage devices and processing circuits to enhance digital video images, it is becoming advantageous to convert the analog signal from the video camera to a digital format for subsequent processing and display. Furthermore, by digitizing the video signal at an early stage in the signal processing, the effects of electrical noise on the signal are reduced. As a consequence, it is advantageous to digitize the X-ray video image signal as soon after the video camera as possible. However, since a high resolution two-dimensional X-ray image can comprise in excess of 4000 by 4000 picture elements (pixels), a conventional digital brightness averaging circuit must be able to process relatively large digital numbers in order to derive an average image brightness value. This results in a relatively complex digital circuit as compared to the analog equivalent.
During fluoroscopic examination of a patient, the physician may insert a lead gloved hand into the field of view to manipulate the patient. In other procedures, an X-ray opaque dye is injected into or ingested by the patient to enhance the contrast of selected anatomical features. Both the lead glove and dye produce extremely black portions of the X-ray image. These "artificially" blackened portions can be interpreted by an automatic brightness control as a darkening of the overall image, thereby misleading the control circuit into correctly adjusting the X-ray exposure. In this situation, the exposure would be misadjusted for imaging the anatomical features which are not covered by the glove or blackened by the dye.
In other situations, the patient may be positioned into only part of the X-ray beam allowing a portion of the beam to pass directly to the image intensifier tube without going through the patient. This portion of the beam is not attenuated significantly and creates a very bright section the resultant image. If the picture elements in the very bright image section are used in the averaging process, the brightness control circuitry may also misadjust the X-ray exposure for imaging the patient's anatomical features.